X-ray spectrochemical analysis is a technique for measuring, both qualitatively and quantitatively, the elemental composition of a substance. In liquid, solid or gaseous form, a continuous spectrum of x-radiation is directed to the surface of the sample which then emits x-rays characteristic of its elemental makeup. The wavelengths or energy pattern emitted, distinct for each element, form the basis for qualitative analysis; their intensity forms the basis for quantitative analysis.
The reliability of quantitative determinations depends on the physical and chemical uniformity of each of the samples and the reference standards. Thus, if a sample is a liquid, its viscosity, temperature, chemical composition, surface, and other physical and chemical parameters must be kept as close to that of the standards to achieve the base accuracy and precision of measurement.
Liquids present a special analytical problem. From long experience, it has been found that the best accuracy and precision are attained when the liquid is analyzed with so-called inverted optics. That is, the x-ray beam is directed upwards against the sample which is contained in a cell or container fitted with a thin window, generally a plastic film, that is virtually transparent to x-rays. For many years, cells for carrying liquids to be analyzed have been commercially available, and these cells are generally inexpensive and are molded from a polymer with a plastic film window stretched into place by a plastic or elastomer ring.
Such cells exhibit limitations especially when, to improve accuracy or lower the limit of detection, the sample is exposed to x-rays for an extended period of time, perhaps several minutes. Under such circumstances, the liquid, absorbing much of the x-rays, heats up and thereby expands in volume. The thin plastic film window then "balloons" and, if heated still further, may actually burst or permit leakage at the seal. In addition, even a small departure from flatness of the plastic film window causes a change in the intensity of x-rays which, in turn, degrades the accuracy of the analysis. To prevent this, it has been a common practice to pierce the floor of the cell, (the top of the cell when it is in the x-ray spectrometer), to permit venting. In some commercial cells, a small portion of the floor is made thinner to facilitate piercing.
At the current state of the art, a tool such as a needle is used by the operator to pierce the floor of the cell to provide the desired venting. It can be seen that a needle, which is separate from the x-ray apparatus itself, may be troublesome to handle, and, in addition, such a needle might be dropped and lost, or other unexpected problems might arise. Also, if the liquid under test has been heated and expanded beyond a critical amount, it may flow over the edge of the floor of the cell, and, depending on the liquid, the x-ray apparatus may be fouled or damaged. Presently available cells are all subject to these problems.